Lightwave communication systems use optical amplifiers to carry large amounts of multiplexed data over long distances from a transmit terminal to a receive terminal. The maximum distance that the data can be transmitted in the fiber without amplification or regeneration is limited by, among other things, loss associated with the optical fiber. To transmit optical signals over long distances, the lightwave systems may include a number of optical amplifiers periodically located along the fiber route from the transmit terminal to the receive terminal. Each optical amplifier boosts the received signal to compensate for transmission losses which occurred since the last repeater. Optical amplifiers are attractive alternatives to other amplifier forms because they amplify lightwave signals in their optical form without conversion into a corresponding electrical signal. Each such optical amplifier is pumped, either optically or electrically, to provide signal amplification.
One optical amplifier realization that is commonly employed in lightwave communication systems is rare-earth doped optical fiber amplifiers. One desirable feature of such amplifiers is the automatic gain control they provide when operating in saturation. However, one disadvantage of such amplifiers is that they yield undesirably high noise levels. In contrast, a Raman amplifier, which is another optical amplifier realization, offers potentially lower noise levels but does not easily provide the gain control.
Doped fiber amplifiers that are pumped by a remotely located optical pump source via the transmission path are known. In such systems Raman amplification also occurs as the pump energy traverses the transmission path. However, remotely pumped systems are used in short-haul, unrepeatered lightwave transmission systems and hence automatic gain control in the amplifiers is not a concern. Accordingly, such systems do not take advantage of the full benefits offered by doped optical amplifiers operating in saturation.
It would therefore be desirable to provide a lightwave communications system which combines the automatic gain control characteristics of doped fiber amplifiers with the low noise characteristics of Raman amplifiers.